It is desirable to use phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) as dielectric layers in the fabrication of integrated circuits. PSG and BPSG have certain advantages over conventional, relatively undoped oxide dielectric layers. For example, their relatively low melting points (approximately 850.degree. to 1200.degree. C.) allows them to be reflowed after deposition to planarize a surface or slope the sidewalls of any contact or via opened in them. This is particularly advantageous for metal interconnects, as a via or contact with sloped sidewalls results in good conformal metal depositions therein and avoids metal electromigration problems. The low temperatures at which these reflows occur allows the reflow step to take place even after the implantation of semiconductor dopant species, as the temperature is not high enough to permit substantial diffusion of these dopants into surrounding areas of the semiconductor.
Another advantage of PSG and BPSG is their ability to scavenge group IA metals such as sodium and potassium. For this reason, it is often used as a passivating barrier.
PSG and BPSG are glasses that typically include approximately 1 to 10% by weight of phosphorus in their chemical formula. The boron in BPSG may constitute up to 5% boron by weight of this glass. The impurities in PSG and BPSG, particularly phosphorus, have a tendency to diffuse into adjoining semiconductor substrates after the application of elevated temperature. Thus, the reflow of PSG cannot be done where the reflowed layer immediately adjoins a semiconductor layer such as silicon without the danger of counterdoping the semiconductor with phosphorus. This problem is conventionally solved by placing a diffusion barrier such as a thin oxide (approximately 1000 Angstroms thick) in between the PSG and the underlying structure.
The necessity of interposing an undoped oxide diffusion barrier layer between the PSG and the underlying integrated circuit structure presents the additional problem of finding an etchant that will selectively etch PSG with high selectivity to the oxide. Without this high selectivity, there is a danger that the undoped oxide diffusion barrier would be compromised.
Various processes are known in the art for etching oxide with high selectivity to silicon. One such chemistry is described in my copending application Ser. No. 755,140, filed Jul. 15, 1985 (Attorney's Docket No. TI-10883). Other examples can be found in Heinecke, 18 Solid State Electronics 1146 (1975); Heinecke, 21 Solid State Technology 104 (1978) and Winters, 49 J. Appld. Phys. 5165 (1978). However, the last three disclosures concern fluorocarbon-based chemistries that introduce the problem of carbon polymerization on the surface. None of the literature discloses or suggests an etchant for etching PSG with high selectivity to oxide, or a PSG etchant that avoids carbon polymerization by the incorporation of an inorganic center. Hence, a need has arisen in the industry for such an etchant.